The present disclosure relates generally to methods and systems for performing acoustic measurements of subterranean formations. More specifically, some aspects disclosed herein are directed to methods and systems that utilize multiple acoustic energy sources in a borehole for characterizing subterranean formations having oil and/or gas deposits therein.
Seismic exploration can provide valuable information useful in the drilling and operation of oil and gas wells. In seismic exploration, energy is introduced by a seismic source to create a seismic signal that is propagated throughout the subterranean formation. This seismic signal is reflected to differing degrees by features that are of interest. A receiver monitors these reflected signals to help generate a seismic map of the underground features. This map is generated by knowing the exact time that a seismic signal was activated as compared to the time that the reflected signal is received. As a practical matter, the system comprises a plurality of sources and receivers to provide the most comprehensive map possible of subterranean features. Different configurations may yield two dimensional or three dimensional results depending on their mode of operation.
As disclosed herein, the subject formations may be saturated with oil and gas deposits including gas hydrates, such as methane hydrates. A gas hydrate is a crystalline solid that is a cage-like lattice of a mechanical intermingling of gas molecules in combination with molecules of water. The name for the parent class of compounds is “clathrates” which comes from the Latin word meaning “to enclose with bars.” The structure is similar to ice but exists at temperatures well above the freezing point of ice. Gas hydrates include carbon dioxide, hydrogen sulfide, and several low carbon number hydrocarbons, including methane. In one aspect, the disclosure herein relates to the recovery of methane from subterranean methane hydrates.
Methane hydrates are known to exist is large quantities in two types of geologic formations: (1) in permafrost regions where cold temperatures exist in shallow sediments and (2) beneath the ocean floor at water depths greater than 500 meters where high pressures prevail. Large deposits of methane hydrates have been located in the United States in Alaska, the west coast from California to Washington, the east coast in water depths of 800 meters, and in the Gulf of Mexico.
A U.S. Geological Survey study estimates that in-place gas resources within gas hydrates consist of about 200,000 trillion cubic feet which dwarfs the previously estimated 1,400 trillion cubic feet of conventional recoverable gas reserves in the United States. Worldwide, estimates of the natural gas potential of gas hydrates approach 400 million trillion cubic feet.
Natural gas is an important energy source in the United States. It is estimated that by 2025 natural gas consumption in the United States will be nearly 31 trillion cubic feet. Given the importance and demand for natural gas the development of new cost-effective sources can be a significant benefit for American consumers.
Gas hydrates dissociate or form when temperature and/or pressure conditions cross the equilibrium border. In considering gas hydrates as an energy resource, understanding those parameters is important for developing efficient production schemes. Producing natural gas from gas hydrates is a technical challenge by itself; it requires substantial engineering effort and a thorough understanding of the behavior of the hydrates underground. The latter becomes more significant for commercial production, which requires stable and well-controlled production methods.
Gas production from hydrates accompanies significant change of petrophysical, geophysical, and geomechanical properties of the hydrate bearing formations due to hydrate dissociation, which is phase transition from solid to a mixture of liquid and gas. In some cases, the formation could collapse because of a lack of strength after the dissociation. To avoid such a situation, and to appropriately manage gas production, it is important to know the range of the hydrate dissociation, i.e., where the dissociation front is.
Although acoustic logging tools are known, there is need for improved methods and systems for acoustically monitoring subterranean formations to derive key parameters relating to the formations. In this, one object of the present disclosure is to provide an improved downhole seismic tool having multiple sources. A tool with a plurality of acoustic sources may be used, for example, to conduct crosswell tomography for enhanced seismic resolution in the formations surrounding the borehole. Another object of the present disclosure is to enable acoustic imaging of subterranean formations with an acoustic tool configured, for example, to monitor the dissociation front in a gas hydrate bearing formation.